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Scientific Papers 



of THE 



Bureau of Standards 

S. W. STRATTON, Director 



No. 408 

effect of the rate of cooling on the 

magnetic and other properties 

of an annealed eutectoid 

carbon steel 



BY 

C. NUSBAUM, Associate Physicist 
W. L. CHENEY, Assistant Physicist 
Bureau of Standards 



JANUARY 22, 1921 



I! i 



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EFFECT OF THE RATE OF COOLING ON THE MAG- 
NETIC AND OTHER PROPERTIES OF AN ANNEALED 
EUTECTOID CARBON STEEL 



By C. Nusbaum and W. L. Cheney 



ABSTRACT 

This paper discusses the results of a study of the effect of the rate of cooling upon 
the magnetic and other properties of an annealed eutectoid steel. In carrying out 
the experiment, six specimens of this steel were selected, heated to 8oo° C, and al- 
lowed to cool at various rates — one in air, another in lime, and the remainder in the 
furnace at slower and slower rates. The following properties were then measured: 
Normal induction for ordinary and large magnetizing forces, residual induction, 
coercive force, resistivity, and Shore scleroscope hardness. In order to study the 
metallographic structure of the steel, micrographs of each specimen were made at a 
magnification of 500 diameters. 

A study of the experimental results show that as the cooling rate is diminished there 
is a pronounced increase in the value of the maximum induction for a given small 
magnetizing force, a decided increase in the maximum permeability, but a decrease 
in the coercive force. The values of the coercive force agree very well with the 
scleroscope hardness number. 

In plotting the reluctivity line as calculated from the. magnetization curve, it is 
found that the reluctivity line consists of- two straight lines with the bend occurring 
at a definite point. As the structure of the steel changes from an essentially sorbitic 
one to "divorced" pearlite, there is a gradual shifting of this bend toward the origin 
and a greater difference between the values of the "real" and "apparent" values 
of the saturation intensity of magnetization as calculated from the slopes of the reluc- 
tivity line. 



CONTENTS 

Page 

I . Introductory 66 

II. Experimental method 66 

1. Material 66 

2 . Heat treatment 67 

3. Magnetic measurements 67 

4. Resistivity 67 

5. Hardness x 67 

III. Observations and results 68 

1. Rates of cooling 68 

2 . Magnetic properties 69 

3. Microscopical analysis 7 2 

IV. Discussion 74 

V. Summary , , , . , .,..,.,, 76 

21623°— 21 65 



66 Scientific Papers of the Bureau of Standards [Vol. i? 

I. INTRODUCTORY 

In a previous paper 1 the effect of the quenchings and subsequent 
tempering on the magnetic properties of a eutectoid carbon steel 
were presented and the metallurgical significance of these effects 
were discussed. Particular use was made of the magnetic re- 
luctivity relationship. The investigation has since been ex- 
tended to slower rates of cooling, some of which are within the 
annealing range. This investigation is discussed in the present 
paper. 

That the physical properties and the crystalline structure of 
steel are greatly affected by various rates of cooling from or 
through the critical range is universally recognized by metallur- 
gists. The influence of the rate of cooling on the magnetic proper- 
ties is also generally recognized, yet there does not appear to be a 
full appreciation of its importance. In much of the published 
magnetic data mention is made that the steel was in the " annealed 
condition," but generally no information is given as to the initial 
temperature, the length of time the material had been kept at 
such temperature, or the rate of cooling. Perhaps the data of 
Yensen 2 on electrolytic iron and silicon steel are the most complete 
in this respect. The present paper is an attempt to emphasize 
the importance of these factors by presenting data on the mag- 
netic and other physical properties and also on the microstructure 
resulting from various cooling rates. 

II. EXPERIMENTAL METHOD 
1. MATERIAL 
The specimens were of the following chemical composition: 

Per cent 

C 0.85 

Mn 23 

Si 23 

Cr 05 

P 016 

S 014 

Fe (by difference) 98. 61 

The specimens were in the form of cylindrical rods 1 cm in diam- 
eter and 25 cm long, and had been previously heated to 8oo° C 
and furnace annealed. 

1 Nusbaum, Cheney, and Scott, B. S. Sci. Papers, No. 404; 1920. 

2 T. D. Yensen, Univ. of Illinois Engineering Experiment Station, Bull. No. 72; 1914. 



ct s n!y m ] Magnetic Properties of Eutectoid Steel 67 

2. HEAT TREATMENT 3 

Each specimen was heated to a temperature of 8oo° C in a small 
Hoskins resistance furnace and kept at this temperature for a 
definite interval of time. The temperatures were measured by 
a platinum platinum-rhodium thermocouple. Oxidation of the 
specimens at the high temperatures was reduced by the presence 
of a gas flame within the furnace. Since, at the slow cooling 
rates, decarburization of the outer portions of the specimen be- 
comes appreciable, all of the specimens were turned down to a 
diameter of 8 mm so that the specimens throughout their entire 
cross section would be uniform in properties and composition. 
One end of each specimen was kept for microscopic examination. 

3. MAGNETIC MEASUREMENTS 

The magnetic measurements at ordinary magnetizing forces 
were made with a Burrows 4 permeameter. At high field values the 
measurements were made by a modified form of the "isthmus" 
method. 5 For the latter measurements it was necessary to reduce 
the diameter of the specimens to 6 mm. 

4. RESISTIVITY 

The resistance measurements were made by a comparison 
method: The "fall of potential," both across the terminals of a 
standard shunt connected in series with the current-carrying 
specimen, and across knife-edge potential terminals in contact 
with the specimen and 20 cm apart, was measured by a high- 
resistance Wolff potentiometer. A series of alternate measure- 
ments were taken. In order to minimize the effects on the speci- 
men of temperature changes caused by the heating effects of the 
current, observations were not taken until an equilibrium tem- 
perature had been reached. 

5. HARDNESS 

One end of each specimen was polished for the determination 
of the scleroscope hardness, and the determinations were made by 
a self-recording Shore scleroscope. The size and form of the 
specimens did not permit of Brinell hardness tests. 

3 The heat treatment was carried out by Messrs. H. Scott and T. G. Digges, of the heat-treating laboratory 
of the Bureau of Standards. Mr. Scott also gave the writers valuable assistance in selecting the most 
suitable cooling rates for these experiments. 

4 C. W. Burrows, B. S. Sci. Papers, No. 117; 1909. 

5 W. I,. Cheney, B. S. Sci. Papers, No. 361; 1920. 



68 Scientific Papers of the Bureau of Standards [Vol. n 

III. OBSERVATIONS AND RESULTS 

1. RATES OF COOLING 

Such rates of cooling were chosen as to cause a gradual varia- 
tion from an essentially sorbitic structure to one in which the 
matrix is essentially divorced pearlite. Specimen 16 was allowed 
to cool in air, and specimen 17 in lime, while specimens 18, 19, 20, 




90 m 150 780 2/0 

Time > minutes 

I^G. 1. — Cooling curves for 4 specimens of annealed eutectoid carbon steel 



270 



and 21 were furnace cooled at various rates and under different 
conditions. In Fig. i are plotted the time-temperature curves 
for the furnace-cooled specimens. In the case of specimen 18 
the current was cut off after the temperature had been held at 
8oo° C for 12 minutes, and the specimen was then allowed to 
cool with the furnace. Specimen ig was held for a time interval 
of 13 minutes at a temperature of 8oo° C, and then slowly cooled 
by reducing the current, which was cut off when the temperature 



Nusbauml 
Cheney J 



Magnetic Properties of Eutectoid Steel 



69 



had fallen to 580 C; the specimen was then allowed to cool with 
the furnace. Specimen 20 was held for a period of 20 minutes at 
the initial temperature and allowed to cool in a definite manner by 
suitable reductions of the current. Specimen 21 was cooled to 
650 C in a definite manner and then held at this temperature for 
a period of 75 minutes in order to determine the effect upon the 
physical properties and structure of maintaining the material 
below its critical temperature during a given time interval. The 
times of cooling for the various specimens from 8oo° C to 650 C 
are recorded in Table 1 . 

TABLE 1. — Effect of Rate of Cooling on Magnetic and Other Properties of Eutectoid 

Carbon Steel 













Magnetic induction 






Satu- 








Time 
held 

at 
800° C 


Method of 
cooling 


Time 
cool- 
ing 
from 
800°- 
650° C 


Maxi- 
mum 
per- 
mea- 
bility 




Resid- 
ual in- 
duction 
H m 150 


Coer- 
cive 
force 

H m 150 


ration 
inten- 
sity of 
mag- 
neti- 
zation 
(calcu- 
lated) 


Scle- 
ro- 
scope 
hard- 
ness 




Spec- 
imen 
num- 
ber 


H=15 


H=150 


H=1000 


Specific 
resist- 
ance 




Min- 
utes 




Min- 
utes 




Gausses 


Gausses 


Gausses 


Gausses 


Gausses 


C. g. s. 

units 




Mi- 
crohms 


16. .. 








385 


5150 


16 100 


20 050 


10 050 


13.4 


| 1564 


I43. 3 


0. 2025 


17 








437 


6150 


16 300 


20 000 


10 540 


12 4 


{ 1520 
f 1550 


>42. 3 


.2006 


18.... 


12 


JF u r n a c e 
) cooled 


| 56 

1 


630 


9300 


16 100 


20 030 


12 300 


10.18 


{ 1500 
| 1580 
{ 1490 
1 1600 
{ 1430 


[•33.0 


.1941 


19.. 


13 


JF u r n a c e 
| cooled 


I 78 


630 


9300 


16 100 


20 030 


12 300 


10.0 


Uo.4 


.1950 


20.... 


20 


JF u r n a c e 
j cooled 


i 200 


698 


10 000 


16 300 


20 000 


12 360 


9.2 


J 1610 
{ 1430 


[29.9 


.1931 


21.... 


13 


JF u r n a c e 
[ cooled 


I 90 


633 


9300 


16 100 


20 080 


11 970 


9.2 


J 1540 
{ 1400 


133.6 


.1963 



2. MAGNETIC PROPERTIES 

Fig. 2 gives the normal induction curves for low and medium 
values of the magnetizing force, and Fig. 4 gives the curves for high 
values. It is interesting to observe the increase in the magnitude 
of the induction for a given value of the magnetizing force, as 
the rate of cooling is varied from air cooling to slow furnace 
cooling. The change in the values of the induction is most marked 
in the magnetizing force intervals, 10 to 20 gausses, and between 
the rates for the lime-cooled and normal furnace-cooled speci- 
mens. The curves for specimens 18, 19, and 21 practically agree 
throughout the entire range of the magnetizing force, and for 
higher values the three corresponding curves are plotted as one 
curve. 



Scientific Papers of the Bureau of Standards 



[Vol. 17 




Magnetizing Force. 



Fig. 2. — Curves showing effect of rate of cooling annealed eutectoid carbon steel upon 
magnetic induction for small and intermediate magnetizing forces 




Fig. 3. 



Id W To IS 3 6 3S To 

Haqtietizinq Force 

-Curves showing effect of rate of cooling annealed eutectoid carbon steel upon 
permeability 



Nusbaum 
Cheney 



Magnetic Properties of Eutectoid Steel 



7i 



The relationship between the magnetizing force and the corre- 
sponding induction can, perhaps, best be noted by plotting the 
permeability against the magnetizing force as the independent 
variable, as shown in Fig. 3. Each curve passes through a maxi- 
mum of increasing magnitude as the rate of cooling of the corre- 
sponding specimen decreases. A continual shifting of this maxi- 




10000 



500 



1000: 



isoo 



1000 



fldgnettjcimj Force 

Fig. 4. — Curves showing effect of rate of cooling annealed eutectoid carbon steel upon 
magnetic induction for large magnetizing forces 

mum toward the origin is also to be noted. The values of the 
maximum permeability are given in Table 1 . 

In Fig. 5 are plotted for the respective specimens the reciprocals 

of the susceptibility, -jr, against the magnetizing force, H, as the 

independent variable. All of the curves have a quite pronounced 
bend in the straight-line portions of the curve. As has been 
shown in a previous paper, 6 this bend is due to the presence of a 
magnetically harder constituent, the cementite in the form of 
conglomerate, stratified, or "divorced" masses. The reciprocal 
of the slope of the lower straight-line portion of the curve is pro- 



• Nusbaum, Cheney, and Scott, B. S. Sci. Papers, No. 404; 1920. 



72 



Scientific Papers of the Bureau of Standards 



[Vol. 17 



portional to the " apparent" maximum intensity of magnetization , 
while the reciprocal of the slope of the upper part of the straight- 
line portion of the curve is proportional to the "real" maximum 
intensity of magnetization of the material for the particular ar- 
rangement of the constituents. Both the apparent and real satura- 
tion values are given in Table i. Attention is called to the 




Z50 4 .500 

Waonetizing Force 



1000 



Fig. 5. — Curves showing effect of rate of cooling annealed eutectoid carbon steel upon 

reciprocal of susceptibility 

gradual shift of the bend in the curves toward lower values of the 
magnetizing force with decreasing values of the rate of cooling. 

3. MICROSCOPICAL ANALYSIS 

The micrographs of Figs. 6 to 11, inclusive, all at a magnification 
of 500 diameters, represent the average structural condition shown 
by etching for 10 seconds in 5 per cent alcoholic picric acid. The 
difference between the furnace-cooled, the air-cooled, and the 
lime-cooled specimens is very marked. The air-cooled material 
(specimen 16, Fig. 6) consists largely of sorbite with intervening 
patches of coarse pearlite. In these patches "■ free " ferrite is seen 

7 This analysis was made by H. S. Rawdon, chief of the metallographic laboratory of the Bureau of 
Standards, to whom the writers are greatly indebted for his painstaking work. 



chen!y m ] Magnetic Properties of Eutectoid Steel 73 




Fig. 6. — Micrograph of specimen 16, cooled in air from 8oo° C 




Fig. 7. — Micrograph of spcimen 17, cooled in lime from 8oo° C 



74 Scientific Papers of the Bureau of Standards [Vol. 17 

to exist, often in masses of considerable size. The effect of the 
slow cooling in lime (specimen 17, Fig. 7) was to allow most of the 
grains, sorbitic when air cooled, to form as lamellar pearlite. 
The patches of coarse pearlite with the conspicuous^ free ferrite 
still persist after cooling in lime. pp 

The four specimens (Nos. 18, 19, 20, and 21, Figs. 8, 9, 10, and 
11), cooled slowly in the furnace, are very similar in structure. 
Patches of lamellar pearlite were found in the structure of each, 
the amount decreasing somewhat as the rate of cooling of the 
specimen was decreased. The matrix of the material in each 
case (specimens 18, 19, 20, and 21) consists largely of divorced 
pearlite; that is, cementite particles in a matrix of ferrite. The 
size of the cementite particles increases somewhat as the rate of 
cooling of the specimen is decreased. 

IV. DISCUSSION 

The magnetization curves for specimens 18 to 21, inclusive, 
are quite characteristic of magnetic materials in the annealed 
condition. This is indicated by the comparatively high values 
of the maximum permeability. The results obtained are similar 
to those of Yensen 8 on electrolytic iron melted in vacuo. In 
his experiments much slower rates of cooling were used. How- 
ever, the effect of magnetizing forces of 15 gausses in the eutectoid 
steel is more pronounced than for corresponding rates of cooling 
in pure iron. This effect is evidently due to the influence of the 
cementite in the resulting sorbitic and pearlitic structures acting 
both as a magnetically harder medium and as a retarding agent 
for the transformation from gamma to alpha iron. The first of 
these is more likely the predominant factor. 

The experiments of Howe 9 show a quite definite relationship 
between the logarithm of the time of cooling from 800 to 650 C 
and the resulting structure of a eutectoid carbon steel. While 
the results here presented show no mathematical relationship 
between the cooling rates and the magnetic constants, there is, 
however, a marked correspondence. The maximum induction 
increases or decreases directly, and the coercive force, inversely, 
with the time of cooling from 800 to 650 C. The residual induc- 
tion does not show as marked a relationship. Since both the 
magnetic constants and the metallographic structure are in- 

8 T. D. Yensen, Univ. of Illinois Engineering Experiment Station Bull. No. 72; 1914. 

9 Howe and Levy, Jour. Iron and Steel Inst., 94, pp. 210-232; 1916. 



Nusbauml 
Cheney J 



Magnetic Properties of Eutectoid Steel 



75 




Fig. 8. — Micrograph of specimen 18, cooled in furnace from 8oo° C 




Fig. 9. — Micrograph of specimen ig, cooled slowly in furnace from 8oo° C 



76 Scientific Papers of the Bureau of Standards [Vol. in- 

fluenced by the rate of cooling, these constants should be useful 
in the application of magnetic tests for determining the quality 
and structure of steel. 

As already stated, the break in the reluctivity line increasingly 
shifts toward the origin with slower cooling rates. In the air- 
cooled specimen, where the structure consists largely of sorbite 
with intervening patches of coarse pearlite, the break occurs at 
the highest value of the magnetizing force. As the material is 
allowed to cool at various rates so as to form a gradation in struc- 
ture from sorbite to divorced pearlite, the break is shifted toward 
lower values of the magnetizing force. There is also a greater 
difference between the real and apparent maximum intensity of 
magnetization. In specimens 18, ig, and 21, where patches of 
lamellar pearlite exist, the shifting of the break toward the origin 
is still more pronounced, all of the breaks occurring at approxi 
mately the same magnitude of the field values. There is also 
the largest difference between the real and apparent values of 
the maximum intensity of magnetization. The break in the 
reluctivity line occurs at the lowest field value in specimen 20, 
where the pearlite is most completely divorced. For a more 
complete degree of "divorcing" (the "spheroidal" state) the 
break would probably occur at still lower values of the magnet- 
izing force.- This shifting is very likely due not only to changes 
in the magnetic properties of the f errite matrix, one of the magnetic 
constituents, but also to the change in the flux distribution 
produced by the change in the arrangement of the cementite 
relative to the ferrite. 

The scleroscope hardness and the resistivity as related to the 
cooling rates are interesting and significant. There is also a 
quite definite relationship between the coercive force and the 
scleroscope hardness, except for specimen 21, which was main- 
tained at 650 C for a definite time. It should be remembered, 
however, that the scleroscope measures essentially a property 
of the material quite near the surface, while the coercive force is 
a property of the material as a whole. 

V. SUMMARY 

In this paper are presented data showing the influence of 
various rates of cooling on the magnetic and other physical 
properties and the resulting metallographic structure of a eutectoid 
carbon steel. The results may be briefly summarized as follows: 



Cheney J 



Magnetic Properties of Eutectoid Steel 



77 




Fig. io. — Micrograph of specimen 20, cooled slowly in furnace from 8oo° C 




Fig. 11. — Micrograph of specimen 21, cooled slowly in furnace from 8oo° C and held at 
650 C for definite period of time 



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78 Scientific Papers of the Bureau of Standards \voi. 17 

1 . With decrease in the cooling rate there is a marked increase 
in the value of the maximum induction for a given value of the 
magnetizing force, an increase in the magnitude of the maximum 
permeability, and a decrease in the magnitude of the coercive 
force. 

2. As the structure is changed from an essentially sorbitic 
one, through lamellar pearlite to divorced pearlite, there is a 
gradual shifting of the break in the reluctivity line toward the 
origin. Also the difference between the magnitudes of the real 
and apparent values of the maximum intensity of magnetiza- 
tion is greatest when the structure is that of lamellar pearlite. 

3. There is a marked agreement between the values of the 
coercive force and the scleroscope hardness, as influenced by 
the various cooling rates, except when the specimen is held at a 
temperature of 650 C for a definite time. 

Washington, October 20, 1920. 



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